If you use individual basins each one must be vented. A single large basin, which is too big for your purposes only has a single vent. Most plumbing codes have the diagram for the setup.

: Thanks for your input, hj! Do the basins need to be vented? I assume there would have to be clean-out access for at least the first 2 basins...I don't anticipate a great deal of waste in the basins, so I wouldn't expect to have to clean-out much at all. Although I believe I've seen this concept before, can you steer me to a plan/blueprint for the triple basin? Also, instead of one large 6" floor drain in center of garage between stalls, would 2 smaller drains (1 under each stall) work as well? My situation would be easier for the 2 drains vs. 1 (I know that sounds like twice the work, but it isn't...to an extent). Thanks again!!

: : : A triple basin is as much a concept as it is a device. It can be one unit similar to a septic tank but divided into three chambers with underwater ports connecting them. The effluent enters the first compartment where heavy sand and silt settle out and some light liquids rise to the surface, it then proceeds to the second basin by way of the port in the dividing wall where finer silt and additional light materials are separated out. Finally it enters the third compartment. If the first two did their job, there is little additional material to separate out in the third compartment, but it is a final line of defense. The water exits the final compartment by means of a trapped pipe which allows the intermediate water, that which is between floating light material and heavy settled material, to flow to the sewage system or dry well. This process was originally done with a set of three cast iron basins linked together and individually vented since each basin would have been double trapped by its own trap and the one in the preceeding basin.: : : What does a triple basin entail, not only for my own sake, but so I'm sure job is done right should I hire someone to do it. I assume a triple basin is needed for sanitary sewer tie-in as well?

: : : : It is not legal since garage run off has grease and oil in it and that will contaminate the soil if it goes to a dry well. Garage runoff is supposed to pass through a triple basin to separate the materials before it enters any drain system.

: : : : : I want to install floor drain in upper level of existing capped garage. I have no way to tie into sanitary sewer. What options do I have? Would a dry well work?